Black background on Alpha
The problem here is Lens Distortion Filter you added in the Compositor. Because these distortions are calculated on color values (especially dispersion), the transparent area is taken as RGB 0/0/0 i.e. black.
To get back the transparency, you could plug the Alpha output of the Render Layers node into the Alpha input of the Composite and Viewer nodes.
Of course in this case, all distortion / dispersion that extends the alpha of the original image will not be visible because it becomes transparent.
You could take the original Alpha and mix in a copy of the Lens Distortion node before plugging it into the Composite/Viewer alpha, this helps but dispersion doesn't look the same as on black background.
See the attached image for comparison:

If you use the original alpha, the silhouette of the cube isn't distorted, the dispersion is almost invisible except for some skewed looking edges.
The alpha with dispersion takes the distortion/dispersion into account and makes the distorted part going from opaque to transparent according to its brightness.
This is how the dispersion normally looks on a black background and how it's calculated on a transparent background.
If you take the result from 2. and alpha over it on a black background it comes close to the real dispersion of number 3. However, it's not as colorful because of the transparency.
Here comes the problem: since real dispersion is splitting up the colors into spectral colors, on a white background it looks quite multicolored.
But if you just take the dispersion from the transparent image and alpha it over a white background, the multicolor is gone since the dispersion only worked on the blue of the cube and not the background.